1. Field of the invention
The present invention relates to a rotation speed control device for a cooling fan of an internal combustion engine in a vehicle such as an automobile, and particularly relates to a rotation speed control device for a hydraulically operated variable speed cooling fan of an internal combustion engine in a vehicle such as an automobile.
2. Description of the prior art
It is already well known in the art of an internal combustion engine in a vehicle such as an automobile to drive a cooling fan which supplies a flow of cold air to an engine cooling radiator by a hydraulic motor, and to control the rotation speed of such a motor according to the coolant temperature and other parameters, as is disclosed in, for example, Japanese Utility Model Publication No. Sho 49-40183 (1974) and Japanese Patent Laying Open Publication No. Sho. 58-13119 (1983).
3. Problem to be solved by the invention
The pump which supplies an operating fluid to the hydraulic motor is coupled to the output shaft of the internal combustion engine and is thus directly driven by the internal combustion engine. For this reason, when the internal combustion engine is operating at high speed the pump is also rotating at high speed and therefore the pressure within the pump is increased. When the pump is operated at high rotation speed under high pressure in this manner, its durability is reduced as compared with the operation at a low speed under the same pressure, and the pump noise is also increased.